Add exclusion for well-known property types

Previously, any valid property was added to the meta-data of the current
group. This can be annoying for types that are not meant to be bound from
a simple string value. ClassLoader is one example.

A list of well-known types has been added: if the property type matches
an element of this list, it is ignored.

Fixes gh-2012

spring-boot-tools/spring-boot-configuration-processor/src/main/java/org/springframework/boot/configurationprocessor/ConfigurationMetadataAnnotationProcessor.java

@@ -75,6 +75,8 @@ public class ConfigurationMetadataAnnotationProcessor extends AbstractProcessor
 
 	private FieldValuesParser fieldValuesParser;
 
+	private ElementExcludeFilter elementExcludeFilter = new ElementExcludeFilter();
+
 	protected String configurationPropertiesAnnotation() {
 		return CONFIGURATION_PROPERTIES_ANNOTATION;
 	}
@@ -177,10 +179,11 @@ public class ConfigurationMetadataAnnotationProcessor extends AbstractProcessor
 			VariableElement field = members.getFields().get(name);
 			Element returnType = this.processingEnv.getTypeUtils().asElement(
 					getter.getReturnType());
+			boolean isExcluded = this.elementExcludeFilter.isExcluded(returnType);
 			boolean isNested = isNested(returnType, field, element);
 			boolean isCollection = this.typeUtils.isCollectionOrMap(getter
 					.getReturnType());
-			if (!isNested && (setter != null || isCollection)) {
+			if (!isExcluded && !isNested && (setter != null || isCollection)) {
 				String dataType = this.typeUtils.getType(getter.getReturnType());
 				String sourceType = this.typeUtils.getType(element);
 				String description = this.typeUtils.getJavaDoc(field);

spring-boot-tools/spring-boot-configuration-processor/src/main/java/org/springframework/boot/configurationprocessor/TypeElementFilter.java

@@ -0,0 +1,52 @@
+/*
+ * Copyright 2012-2014 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.configurationprocessor;
+
+import java.util.HashSet;
+import java.util.Set;
+
+import javax.lang.model.element.Element;
+
+/**
+ * Filter to excluded elements that don't make sense to process.
+ *
+ * @author Stephane Nicoll
+ * @since 1.2.0
+ */
+class ElementExcludeFilter {
+
+	private final Set<String> excludes = new HashSet<String>();
+
+	public ElementExcludeFilter() {
+		add("java.io.Writer");
+		add("java.io.PrintWriter");
+		add("javax.sql.DataSource");
+		add("java.lang.ClassLoader");
+	}
+
+	private void add(String className) {
+		this.excludes.add(className);
+	}
+
+	public boolean isExcluded(Element element) {
+		if (element == null) {
+			return false;
+		}
+		return this.excludes.contains(element.toString());
+	}
+
+}